1. Field of the Invention
The present invention relates to an apparatus and a method for x-ray diffraction analysis of materials, e.g., semiconductor materials or other crystalline materials, using a photo-sensor fiber-optic stress analysis system.
2. Description of the Related Art
X-ray diffraction is a well known technique for measuring stress in solid materials. X-rays diffracted from the surface of a material provide direct information about the spacing between particular atomic planes of the material. X-rays impinging on a set of atomic planes will scatter in all directions. Constructive interference of the scattering x-rays occurs only at particular angles that the scattering x-rays make with the particular atomic planes, and is dependent on the spacing of the particular atomic planes. This atomic spacing information is represented in the form of diffracted x-ray intensity versus the diffraction angle from the particular atomic planes of the material, and is governed by the Bragg Equation expressed as: EQU .lambda.=2d sin.theta.
where
.lambda.=wavelength of the diffracted radiation PA1 d=atomic lattice spacing of a particular set of atomic planes PA1 .theta.=Bragg angle.
Constructive interference of scattered x-rays occurs when the incident angle of the x-ray beam with the particular set of atomic planes is equal to the diffracted angle, that angle .theta. (referred to as the Bragg angle) satisfying the Bragg Equation. Constructive interference results in intensity maxima, also referred to as diffraction peaks. Each particular set of atomic planes of a material will have an associated diffraction peak which occurs at a particular .theta. angle which satisfies the Bragg Equation. It is common to refer to the 2.theta. angle where the factor of two accounts for both the incident x-ray beam angle with the particular atomic planes, and the diffracted beam angle with the same atomic planes. When a region of material exhibits residual stress, or is under applied stress, the spacing of the atomic planes can increase or decrease with respect to the spacing of the atomic planes in the unstressed state. Thus according to the Bragg Equation when a material is stressed the diffraction peaks will shift to a slightly different Bragg angle, i.e., the .theta. value changes slightly due to a change in spacing of the atomic planes. X-ray diffraction can be used to directly determine the angular shift of the diffraction peak. The change in atomic spacing can be used to determine the strain of the region of material being examined. Stress is subsequently calculated from the strain using the elastic constants of the material.
The preferred atomic planes for x-ray diffraction stress analysis are those which occur at higher diffraction angles, or correspondingly at lower atomic spacing. The x-ray diffraction peaks (or atomic planes) at these higher angles are generally of low intensity compared to diffraction peaks occurring at lower diffraction angles, but they exhibit a larger position shift because of changes in stress than the low angle peaks. Thus the high angle peaks provide a greater degree of accuracy in determining strain and stress. The preferred high angle diffraction peaks for stress analysis are also referred to as high backreflected peaks and generally occur at a 2.theta. angle of greater than 100.degree., and preferably in the range of 130.degree. to 165.degree..
Instruments used to measure stress by x-ray diffraction typically employ a monochromatic x-ray source and either a mechanical scanning point detector or one or more position sensitive detectors. These systems all determine the diffraction peak locations, and the change in the peak location due to stress. The x-ray source, the material being examined, and the detector(s) are arranged in controlled angular relationships in order to measure stress. A thorough description of stress analysis by x-ray diffraction can be found in a book titled iResidual Stress, Measurement by Diffraction and Interpretationi, authored by I. C. Noyan and J. B. Cohen (Published by Springer-Verlag 1987, ISBN 0-387-96378-2).
Fiber-optic based position sensitive detectors used for stress analysis are described in U.S. Pat. Nos. 4,686,631; 5,148,458; and 5,414,747, issued to Ruud et al.; in U.S. Pat. No. 4,489,425, issued to Borgonovi; and in U.S. Pat. No. 5,125,016, issued to Korhonen.
U.S. Pat. No. 4,686,631 discloses and claims a method for obtaining calibration coefficients for residual internal stress measurements in polycrystalline specimens without the requirement of accurately controlling or auxiliarily measuring specimen-to-detector distance in an x-ray diffraction system for stress analysis. The patent discloses single exposure technique (SET) and multiple exposure technique (MET) internal stress measurement methods as well as methodology for verification of coefficient validity. The disclosed technique for obtaining calibration coefficients for stress analysis is described with reference to the use of a multi-channel x-ray detector, which may be a position sensitive detector, a position sensitive proportional detector, or a position sensitive scintillation detector.
In the position sensitive scintillation detector, as disclosed in U.S. Pat. No. 4,686,631, thin scintillation coatings placed on the input face of fiber-optic channel bundles convert diffracted x-rays into visible light and transmit the visible light into the fiber-optic bundles. The fiber-optic bundles then transmit the visible light, derived from diffracted x-ray radiation, to separate photodiode arrays (PDAs). The photodiode arrays are a part of a scanner subassembly typified by an individual linear photodiode array including 512 pixels arranged in a single continuous row of 512 pixels long and one pixel wide. Each fiber-optic bundle transmits light to a separate linear diode array. The linear diodes are arranged in proximity such that they fit within the active area of a circular image intensifier. The individual diodes and associated precharged capacitors are arranged in a face-to-face relationship to the intensified image being received from the corresponding fiber-optic bundles. The photodiodes are responsive to visible light for storing in the capacitors an electronic signal determined to have a functional relationship to the visible light impinging upon the photodiodes. The output of each diode array represents the intensity of the diffracted x-ray beam as a function of the diffraction angle, thus showing the diffraction peak location and shape (referred to herein as a one dimensional diffraction spectrum) from each fiber-optic bundle. The diffraction peak location provided by each of the fiber-optic bundles when analyzed together with data from unstressed material enables determination of the stress in the material.
U.S. Pat. No. 5,148,458 discloses a system for simultaneous measurement of phase composition and residual stress, utilizing an x-ray detector which may be of the type disclosed and claimed in U.S. Pat. No. 4,686,631. There are three position-sensitive x-ray detectors fixedly mounted along a circular arc, an x-ray source for impingement of a beam of x-rays on the center of the circle represented by the arc, and distribution spectra analysis apparatus. A method described includes directing a beam of x-rays on a polycrystalline sample and receiving and detecting resultant Bragg diffraction x-rays by three stationary position-sensitive x-ray detectors located along a circular arc. The sample is located at the center of the circle of the arc. Diffracted x-rays received by two of the detectors are analyzed to determine residual stress, and diffracted x-rays received by the third detector are analyzed to determine phase composition of the sample.
Both U.S. Pat. Nos. 4,686,631 and 5,148,458, describe a linear photodiode array as a position-sensitive detector in their systems. Photodiode arrays, however, have a significant shortcoming as photo-sensor array detectors for fiber-optic stress analysis systems. The photodiode arrays have a large electronic charge storage capacity but that is accompanied by undesirably high background noise levels (dark current and read noise). This makes them optimal for measuring higher light intensities, such light intensity being much greater than the background noise created in the diodes. The diffracted x-ray peaks of many materials and structures used in stress analysis are often very weak, i.e., the peak-to-background signal ratio is low, and the overall diffracted x-ray intensity is much lower than the incident x-ray beam intensity. The scintillation process might typically be only 10% efficient, thus the resultant intensity of the light image entering the fiber-optic bundles is extremely weak, and the intensity of the light peak representing the corresponding x-ray diffraction peak is only slightly greater than the background light intensity on either side of the light peak. To compensate for the extremely weak incoming light signal, an image intensifier is required for PDAs, however, the intensifier magnifies the background light on either side of the diffraction light peak as well. When the high internal noise levels of the PDAs are added to the incoming intensified light signal, the total background light in the final output spectrum will further reduce the peak-to-background signal ratio. Very long data acquisition times are often required for such weak diffraction peaks, and in some cases the peaks are never observed regardless of the acquisition time. The PDAs are also limited in available sizes and in the total amount of PDA active area that can be mated with a single image intensifier.
U.S. Pat. No. 5,125,016 discloses a procedure and two apparatus variations for measuring stress based on x-ray diffraction. One apparatus includes two fiber-optic light cables, each covered with a thin scintillation coating and connected to image intensified silicon diode arrays. The thin scintillation coatings placed on the input face of fiber-optic light cables convert diffracted x-rays into visible light and transmit the visible light first through an image intensifier, then on to a series of silicon photo-diodes. The data from the silicon photo-diodes fed to an analog to digital converter, then to a multichannel analyzer and a computer for graphical display. The fiber-optic light cables are rectangular with, e.g., a cross-section of 1 millimeter by 2 millimeters. In the second apparatus, a linear series of photo-diodes coated with a fluorescent film are employed as the detector, without fiber-optic light cables interposed between the fluorescent film and the diode array.
U.S. Pat. No. 5,414,747 discloses a system for real-time analysis of a plated specimen, to determine analytically variables such as: composition of the substrate and plating; plating thickness; plating depth (under an overlayer); analysis of crystal phase depth concurrently with phase composition; preferred crystalline orientation; substrate strain; crystallinity; and grain size. The apparatus includes a plurality of position-sensitive detector surfaces, an x-ray source for impinging x-rays on a plated specimen, and plural detector surfaces positioned on either side of the incident x-ray beam from the x-ray source, so that the detector surfaces detect x-rays diffracted from a plurality of crystallographic planes of the plated specimen at various angles. The first and second detector surfaces are positioned at different distances from the specimen, and means are provided for analyzing the spectra of diffracted peaks of x-radiation diffracted from the plural crystallographic planes of the plated specimen.
U.S. Pat. No. 5,414,747 also discloses that two fiber-optic detectors may be coupled by an optical amplifier to silicon diode arrays. The diode arrays convert the amplified coherent optical signals into electrical signals which are then transmitted to a central processor or computer for spectra analysis. The patent notes that other types of position-sensitive detectors may be employed, such as charge-coupled devices (CCD) or other one-dimensional or two-dimensional x-ray sensitive devices. There is no description of how to arrange and operate the system with a CCD device to produce a readable signal. Numerous changes must be made for such a system to be operable, and to further operate it in an optimized fashion. Direct replacement of a PDA by a CCD does not appear to be possible.
Most commercial CCDs will not be optimal, nor even adequate for replacing the diode arrays of the x-ray diffraction system taught by U.S. Pat. No. 5,414,747. It is not at all clear from the disclosure of U.S. Pat. No. 5,414,747 which CCDs will work or not and how those that might work should be arranged for operation. Most commercial CCDs are lens-coupled, with fixed shutter speeds and unreasonably high noise levels for low light scientific applications. Newer scientific slow scan image intensified CCDs offer extremely high sensitivity, and controllable shutter speeds, but almost all image intensified CCDs are isolated by a lens coupling and are evacuated for low temperature operation in order to reduce the noise. Lens coupled CCDs cannot be directly coupled to fiber-optic bundles. Scientific slow scan CCDs that are not image intensified are often fiber-optically coupled, but they do not offer the high level of sensitivity preferred for x-ray diffraction stress analysis that image intensified CCDs offer.
PDAs used for stress analysis are one dimensional arrays as disclosed in U.S. Pat. Nos. 4,686,631, 5,148,458, 5,414,747, and 5,125,016. The unmodified graphical output from PDAs represents a one dimensional diffraction spectrum (intensity versus diffraction angle) showing the diffraction peak location, intensity and shape. The diffraction peak is shown on a graphical plot of diffracted intensity in the vertical direction, or y scale, versus the diffraction angle in the horizontal direction, or x scale. A stress in the material being examined will give rise to a shift in the diffraction angle (or position) of the diffracted x-ray peak from its unstressed position. To accurately determine a very small position shift in the diffraction peak, a mathematical curve fitting procedure is typically used to precisely determine the exact angular location of the peak. Desirable CCDs for stress analysis are two dimensional arrays and do not output data for a one dimensional diffraction spectrum, rather they output data for a two dimensional image which cannot be processed by the teachings of U.S. Pat. Nos. 4,686,631, 5,148,458, 5,414,747, and 5,125,016.
U.S. Pat. No. 4,489,425 describes an x-ray diffraction apparatus for making stress measurements using a single position sensitive area detector which can capture x-rays over a large planar region. The apparatus discloses a single rigid fiber-optic tapered array with a large two dimensional planar input area coated with a scintillating material. X-rays are diffracted from the sample surface onto a large planar face input area of the detector and are converted to a visible light image which is then transported to the CCD. The smaller output end of the fiber-optic taper is coupled to a two dimensional CCD and produces a two dimensional image representing a planar intersection of the full x-ray diffraction cone, or a major portion thereof, for subsequent stress analysis. The image consists of an ellipse, or major portion thereof, the total shape of which is analyzed to provide stress information. The large planar face input area of the detector makes it difficult to achieve and control the desired series of angular relationships between the detector, x-ray source and material being examined.
It would therefore be a significant advance in the art, and is an object of the present invention, to provide for the x-ray diffraction analysis of materials, an improved x-ray detector system which overcomes deficiencies of the prior art.